Influence of active filler type and amount on mechanical properties and performance of cold recycled asphalt mixtures

Cold recycling techniques have gained increasing attention over the past few decades as sustainable and cost-effective solutions for pavement rehabilitation. However, the design of cold recycled asphalt mixtures (CRMs) remains challenging, mainly due to the limited understanding of how active fillers influence binder dispersion, cohesion development, and long-term mechanical performance. In particular, the lack of clear criteria for selecting the optimal filler type and dosage hinders consistent field application and performance prediction. This study investigates the influence of active fillers on the mechanical behavior of CRMs through a comparative experimental program. CRM specimens were prepared using either foamed bitumen or bitumen emulsion and incorporating two commonly used active fillers, cement and lime, at different dosages. Two mechanical tests were conducted to evaluate mechanical properties: the indirect tensile strength (ITS) test and the monotonic triaxial shear strength (TSS) test. Additionally, a shear stress to strength ratio analysis was performed to comparatively evaluate the rutting potential under traffic loading. Results indicate that both filler type and amount significantly affect mechanical response. ITS was more sensitive to early binder-aggregate interaction, while TSS better reflected the long-term effects of filler-binder interaction. Cement contents slightly above the commonly recommended 1% threshold may improve the mechanical response, increasing stiffening without clear evidence of brittle behavior. In contrast, lime was effective only at 2% or higher, with 1% proving insufficient for enhancing cohesion or activating pozzolanic reactions. These findings provide preliminary but meaningful insights into the role of active fillers, and their relative influence on the mechanical behavior of cold recycled mixtures.